Further to the comments on the SG6042 vs the Clark Y: The SG6042 is a bit thinner (memory says something like 11% vs 12+% for the Clark Y). Since autorotation is dependent on having minimum drag, the thinner profile helps.

However the profiles of the front half of the Clark Y and the 6042 are almost superimposable. (Except the greater thinness of the 6042.) The slight undercamber of the 6042 toward the trailing edge is clearly apparent.

Bill